How to merge adjacent cubes
What you need to do is select Wall_1_Room04, then select Wall_1_Room05. once they are both selected, with Wall_1_Room05 being the active object press Ctrl+J. This will join the walls, the origin will be at the location of Wall_1_Room05's previous origin.

After that, tab into edit mode, select all and press Ctrl+V > Remove doubles. repeat the process for Wall_7_Room_04.